PSI - Issue 62

Matteo Nicolini et al. / Procedia Structural Integrity 62 (2024) 601–608 Matteo Nicolini/ Structural Integrity Procedia 00 (2019) 000 – 000

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accumulation zone just downstream, which resulted in a flow bifurcation and in lateral erosion of river banks. At the same time, although with a slower temporal dynamics, the backward propagation of the erosion reached the area of the bridges, determining a progressive general lowering of the river bed. As a consequence, many bridge piers of the old bridge resulted over-excavated: first the footings and then the piles came into light, determining severe conditions for the stability of the structure (Fig. 5b,c,d). Fig. 6 shows a succession of images at different times as derived from the results obtained by the simulation model, in order to illustrate qualitatively the dynamics of the collapse and its consequences. Further calculation of the return period provided an estimate of nearly 20 years.

Fig. 6. Results obtained from the morphodynamic model showing the deposition/erosion patterns at different times (hours): the erosive trend propagating upstream is evident, with the progressive excavation of the bridge piers. 4. The restoration phase The morphodynamic model was also a fundamental physics-based support tool for the restoration phase. In particular, the design of the retrofitting strategy was guided by the results obtained through numerical simulations of different events, whose flow hydrographs were provided by the River Basin Authority for the return periods of 30, 100 e 300 years (Fig. 7). Three scenarios were considered in the simulations: the current state (SDF), with the check-dam partially collapsed and the presence of the 'old bridge'; the project scenario (SDP), with the restoration of the check-dam and also considering the elimination of the old bridge; the 'state at that time' (SDA), namely before the check-dam collapse, introduced in order to compare the results of the project scenario with that of the previous condition, i.e. before the collapse, and with the presence of both the new and the old bridge. This was necessary because during the design phase the River Basin Authority required the proof of the small alteration of the hydrodynamic fields as a consequence of the check-dam restoration. In order to prove this, a comparison between the SDP and SDF water depths and flow velocities would not have been the correct approach, thus requiring the differential comparison between the SDP and the SDA. Fig. 8 shows the results obtained in terms of depth and velocity fields, both for the SDF and SDP. As mentioned, the difference between the two situations does not appear evident, but taking the differential maps between SDP and SDF (Fig. 9a,c) shows the high alteration of the hydrodynamic and morphodynamic fields. This does not emerge when the difference between the SDP and SDA situation is plotted (Fig. 9b,d), proving that the project scenario can be considered invariant with respect to the previous configuration.